The present invention relates to an insulated switching power supply for supplying DC power from a bridge-rectified output-smoothed DC voltage source through a transformer and a switching element to a load, and more particularly, relates to techniques for improving an input power factor and for decreasing switching loss and steady-state loss of a switching power supply.
FIG. 5 shows a configuration of a conventional switching power supply that uses a typical choke input-type smoothing circuit to improve the power factor (refer to Japanese Patent Publication (Tokkai) No. 09-131055).
In FIG. 5, a reference number 1 designates an AC supply, reference numbers 2 and 4 designate capacitors, a reference number 3 designates a common mode reactor having two coils wound with the same polarity around the same core, a reference number 5 designates a bridge rectifying circuit, a reference number 6 designates a smoothing capacitor, a reference number 7 designates a transformer, a reference number 7a designates a primary winding, a reference number 7b designates a secondary winding, a reference number 8 designates a switching element, a reference number 9 designates a diode, a reference number 10 designates a smoothing capacitor, a reference number 11 designates a load, a reference number 12 designates a control circuit for controlling a switching operation of the switching element 8, and a reference number 13 designates a choke coil.
Here, the capacitors 2 and 4, together with the common mode reactor 3, form a common mode noise filter. The noise filters which omit the capacitor 2 or capacitor 4 from the above described circuit configuration are well known to those skilled in the art. The noise filter which has any of the circuit configurations described above is called the xe2x80x9cnormal mode noise filterxe2x80x9d. The normal mode noise filter removes the normal mode noise current which flows through the positive and negative output lines of the bridge rectifying circuit 5. For simply forming a normal mode noise filter, a reactor having one single winding may be used in substitution for common made reactor 3.
Although not illustrated, a common mode noise filter is formed by grounding each winding of the common mode reactor 3 via a capacitor. The common mode noise filter is used to remove the common mode noise current which flows between the positive and negative output lines of the bridge rectifying circuit 5 and the ground concurrent with the switching operation of switching element 8.
In the abovementioned configuration, a choke input-type smoothing circuit comprised of the choke coil 13 and the smoothing capacitor 6 smoothes a full-wave rectified DC voltage output from the bridge rectifying circuit 5. Then, through the switching operation of the switching element 8, the voltage is supplied through the transformer 7, the diode 9, and the smoothing capacitor 10 to the load 11 as a nearly constant DC voltage. The control circuit 12 controls an ON/OFF duty cycle of the switching element 8 with pulse-width modulation (PWM) or other means so as to adjust the aforementioned DC voltage at a desired value.
A charging current is supplied to the smoothing capacitor 6 from the AC supply 1 through the noise filter, the bridge rectifying circuit 5, and the choke coil 13. According to an inductance value of the choke coil 13, a peak value of the charging current is suppressed, and a conduction time is increased. Namely, the choke coil 13 smoothes the charging current flowing to the smoothing capacitor 6, thereby improving the power factor (Japanese Patent Publication No. 9-131055).
However, since this switching power supply is used at a commercial switching frequency or twice that frequency, the choke coil 13 requires a large inductance of at least several mH, resulting in a large size and a heavy weight not suitable for practical applications.
Moreover, because of the large inductance, the number of coil windings increases. Accordingly, a voltage drop due to the resistance of the windings becomes larger and an intermediate DC voltage decreases. Therefore, an effective current flowing through the switching element increases, thereby causing problems such as an increased switching loss and decreased efficiency of the power supply.
As shown in FIG. 6, to achieve a power factor of nearly 1 and remove high frequency components from the input current, it is known that a switching power supply may use a method known as power factor correction (PFC) to convert an input current into an approximate sine wave.
In FIG. 6, a reference number 14 designates the second switching element, a reference number 15 designates a diode, a reference number 16 designates a current sense resistor, a reference number 17 designates an inductor, a reference number 18 designates the second control circuit, and other circuit elements are the same as in FIG. 5. Here, a voltage of the smoothing capacitor 6 and a current value sensed by the current sense resistor 16 are input to the second control circuit 18. The second switching element 14 is turned on and off based on these input signals.
In this example of the prior art, the inductor 17, the second switching element 14, the diode 15, the smoothing capacitor 6, the current sense resistor 16, and the second control circuit 18 form a boost converter. The PWM control of the switching element 14 by the control circuit 18 makes the input current waveform sinusoidal to remove the high frequency components and improve the power factor.
The switching power supply shown in FIG. 6 requires two control circuits 12 and 18, and consequently has a complex circuit configuration and a high cost. Moreover, not many applications need to remove all of the high frequency components contained in the input current while maintaining a power factor of nearly 1. For most applications, since it is sufficient to suppress the high frequency components to a set value or below as specified by a specification or the like, this switching power supply is also wasteful in regard to functionality and a cost.
Therefore, an object of the present invention is to provide a switching power supply that improves the power factor by expanding the input current conduction angle over a wide range of the input voltage, removes the high frequency components of the input current to a level sufficient for a practical use, reduces the switching loss, and increases efficiency without increasing a size and a cost of a device.
Further objects and advantages of the invention will be apparent from the following description of the invention.
To solve the abovementioned problems, according to the first aspect of the invention, a switching power supply includes a noise filter; a bridge rectifying circuit connected to an AC power supply via the noise filter; a smoothing capacitor connected to an output side of the bridge rectifying circuit for smoothing a voltage output from the bridge rectifying circuit; a primary winding of a transformer and a switching element connected in series to one terminal of the smoothing capacitor; and a control circuit for controlling a switching operation of the switching element to adjust a DC voltage obtained by rectifying and smoothing the voltage on a secondary winding side of the transformer at a desired value based on a detected value of the DC voltage.
Further, the first reactor and the first diode are connected sequentially in series between one output terminal of the bridge rectifying circuit and one terminal of the smoothing capacitor. The second reactor and the second diode are connected in series between a node that mutually connects the first reactor and the first diode and a node that mutually connects the primary winding and the switching element.
According to the second aspect of the invention, a switching power supply includes a noise filter; a bridge rectifying circuit connected to an AC power supply via the noise filter; a smoothing capacitor connected to an output side of the bridge rectifying circuit for smoothing a voltage output from the bridge rectifying circuit; a primary winding of a transformer and a switching element connected in series to one terminal of the smoothing capacitor; and a control circuit for controlling a switching operation of the switching element to adjust a DC voltage obtained by rectifying and smoothing the voltage on a secondary winding side of the transformer at a desired value based on a detected value of the DC voltage.
Further, the first reactor is connected to a front of the bridge rectifying circuit. An anode of a diode is connected to each input terminal of the bridge rectifying circuit. The second reactor is connected between cathodes of these diodes and a node that mutually connects the primary winding and the switching element.
According to the third aspect of the invention, a switching power supply includes a noise filter; a bridge rectifying circuit connected to an AC power supply via the noise filter; a smoothing capacitor connected to an output side of the bridge rectifying circuit for smoothing a voltage output from the bridge rectifying circuit; a primary winding of a transformer and a main switching element connected in series to one terminal of the smoothing capacitor; and a control circuit for controlling a switching operation of the switching element to adjust a DC voltage obtained by rectifying and smoothing the voltage on a secondary winding side of the transformer at a desired value based on a detected value of the DC voltage.
Further, the first reactor and the first diode are connected sequentially in series between one output terminal of the bridge rectifying circuit and one terminal of the smoothing capacitor. The second reactor and the second diode are connected in series between a node that mutually connects the first reactor and the first diode and a node that mutually connects the primary winding and the main switching element.
Also, a snubber capacitor and the first feedback diode are each connected in parallel to the main switching element. Moreover, in order to) discharge the snubber capacitor, a closed loop circuit is formed of the snubber capacitor, a tertiary winding of the transformer, a diode, and an auxiliary switching element. The second feedback diode is connected parallel to the auxiliary switching element.
According to the fourth aspect of the invention, a switching power supply includes a noise filter; a bridge rectifying circuit connected to an AC power supply via the noise filter; a smoothing capacitor connected to an output side of the bridge rectifying circuit for smoothing a voltage output from the bridge rectifying circuit; a primary winding of a transformer and a main switching element connected in series to both terminals of the smoothing capacitor; and a control circuit for controlling a switching operation of the switching element to adjust a DC voltage obtained by rectifying and smoothing the voltage on a secondary winding side of the transformer at a desired value based on a detected value of the DC voltage.
Further, the first reactor is connected to a front of the bridge rectifying circuit. An anode of a diode is connected to each input terminal of the bridge rectifying circuit. The second reactor is connected between cathodes of these diodes and a node that mutually connects the primary winding and the main switching element.
Also, a snubber capacitor and the first feedback diode are connected parallel to the main switching element. Moreover, in order to discharge the snubber capacitor, a closed loop circuit is formed of the snubber capacitor, a tertiary winding of the transformer, a diode, and an auxiliary switching element. The second feedback diode is connected parallel to the auxiliary switching element.
According to the fifth aspect of the invention, in a switching power supply according to the second and fourth aspects, the noise filter includes a common mode reactor, and a leakage inductance thereof is replaced with the first reactor.